REST api v3: Create Pull Request not working anymore?

I was working on creating some kind of automation on Github with merging PRs (pretty much just trying to trigger a new Pull Request between staging and testing server each time staging is deployed). 

The API in general is working fine with my access key etc, I can view open PRs in the same org_name and repo_name that I am using for POST/Create PR with no problems. But when I try to create a new pull request I keep getting this error: 

“message”: “Invalid request. For ‘links/0/schema’, nil is not an object.”

and request I am trying with CURL or Postman is (POST ofc) and this matches docs for in my opinion. 

Has the API changed without the documentation being noticed and this feature is now only supported by the v4 GraphQL API or am I just doing something wrong here?

edit: used hub to do this eventually.

@exhibiton , I had a similar problem with python requests module and when I used the json keyword over params keyword, it worked. So, the problem is the data you are posting as the status code indicates.